The present invention relates to a sensor coil for discriminating a coin, in particular to a sensor coil for use in coin acceptors or rejectors installed in coil operated vending machines, game machines or other equipment or machines for discriminating a genuine coin or coins of a particular denomination from spurious coins or slugs.
Such a sensor coil has been used in the form of a proximity switch wherein one sensor coil is arranged on one side of a coin passage or a slot switch wherein two sensor coils are opposedly arranged on both sides of the coil passage. The sensor coil may be connected with an oscillator circuit, a rectifier circuit, a Schmidt circuit, an output circuit or other circuit such that when a magnetic field generated by the sensor coil is crossed by a coin which is passing through the coin passage, an induction of the coil varies and this variation of the induction is detected to discriminate the material or the dimension of the coin.
Heretofore, such a sensor coil used in the aforementioned manner has been generally formed in the form of a circular sensor coil as described in Japanese Utility Model Publication No. 56-12,693 or in the form of an elliptical sensor coil as described in Japanese Utility Model Laid-Open No. 53-56,897.
It was found, however, that it is very hard for such known circular and elliptical sensor coils to correctly discriminate coins having different diameters, even if coins are made of the same material.
That is, differences between variations of reactance of the circular sensor coil caused when coins A, B and C of different diameters but the same material cross the magnetic field of the circular sensor coil (see FIG. 6) are expressed only by differences in height and width of the peak variations of variation curves "a", "b" and "c" of the similar shape (see FIG. 7). Further, the peak variations of the coil reactance caused by the coins A, B and C having different diameters occur at the same time position on a time coordinate axis. Furthermore, if the diameter and position of the circular coil are not pertinent for all coins to be discriminated, the peak variations can not be distinguished from each other as is noted by comparing the variation curves "b" and "c" (FIG. 7), resulting in that the coins B and C can not be discriminated by the peak variations.
The elliptical sensor coil (see FIG. 8) also has drawbacks similar to the circular sensor coil since the peak variations of the coil reactance caused by different coins A, B and C occur in the same time position except that the peak variations have increased different widths (see variation curves "a", "b" and "c" in FIG. 9).
With the aim of removing the drawbacks of the above circular sensor coil, Japanese Utility Model Publication No. 56-12,693 proposes an arrangement of a coin position detecting means at a position away from the center of the circular sensor coil in the left or right direction along a coin guide rail in the coil passage to ensure a difference between variations of the coil reactance caused by coins having different diameters.
However, the difference between variations of the coin reactance caused by coins having different diameters is provided at substantially the same time position and is relatively small. Accordingly, high accuracy of measurement is required for surely detecting such a small difference between the variations.
Moreover, in order to correctly discriminate coins having different diameters by use of the circular or the elliptical sensor coil, the coins to be discriminated should be made of substantially the same material. It is therefore necessary to provide an additional sensor coil or coils for selecting coins of the same material.